Structs

Structure A template or pattern given to a logically related group of variables. field - structure member containing data Program access to a structure: entire structure as a complete unit individual fields Useful way to pass multiple related arguments to a procedure example: file directory information Irvine, Kip R. Assembly Language for x86 Processors 7/e, 2015.

Using a Structure Using a structure involves three sequential steps: 1. Define the structure. 2. Declare one or more variables of the structure type, called structure variables. 3. Write runtime instructions that access the structure. Irvine, Kip R. Assembly Language for x86 Processors 7/e, 2015.

Structure Definition Syntax name STRUCT field-declarations name ENDS Field-declarations are identical to variable declarations Irvine, Kip R. Assembly Language for x86 Processors 7/e, 2015.

COORD Structure The COORD structure used by the MS-Windows programming library identifies X and Y screen coordinates COORD STRUCT X WORD ? ; offset 00 Y WORD ? ; offset 02 COORD ENDS Irvine, Kip R. Assembly Language for x86 Processors 7/e, 2015.

Employee Structure A structure is ideal for combining fields of different types: Employee STRUCT IdNum BYTE "000000000" LastName BYTE 30 DUP(0) Years WORD 0 SalaryHistory DWORD 0,0,0,0 Employee ENDS Irvine, Kip R. Assembly Language for x86 Processors 7/e, 2015.

Declaring Structure Variables Structure name is a user-defined type Insert replacement initializers between brackets: < . . . > Empty brackets <> retain the structure's default field initializers Examples: .data point1 COORD <5,10> point2 COORD <> worker Employee <> Irvine, Kip R. Assembly Language for x86 Processors 7/e, 2015.

Initializing Array Fields Use the DUP operator to initialize one or more elements of an array field: .data emp Employee <,,,2 DUP(20000)> Irvine, Kip R. Assembly Language for x86 Processors 7/e, 2015.

Array of Structures An array of structure objects can be defined using the DUP operator. Initializers can be used NumPoints = 3 AllPoints COORD NumPoints DUP(<0,0>) RD_Dept Employee 20 DUP(<>) accounting Employee 10 DUP(<,,,4 DUP(20000) >) Irvine, Kip R. Assembly Language for x86 Processors 7/e, 2015.

Referencing Structure Variables Employee STRUCT ; bytes IdNum BYTE "000000000" ; 9 LastName BYTE 30 DUP(0) ; 30 Years WORD 0 ; 2 SalaryHistory DWORD 0,0,0,0 ; 16 Employee ENDS ; 57 .data worker Employee <> mov eax,TYPE Employee ; 57 mov eax,SIZEOF Employee ; 57 mov eax,SIZEOF worker ; 57 mov eax,TYPE Employee.SalaryHistory ; 4 mov eax,LENGTHOF Employee.SalaryHistory ; 4 mov eax,SIZEOF Employee.SalaryHistory ; 16 Irvine, Kip R. Assembly Language for x86 Processors 7/e, 2015.

. . . continued mov dx,worker.Years mov worker.SalaryHistory,20000 ; first salary mov worker.SalaryHistory+4,30000 ; second salary mov edx,OFFSET worker.LastName mov esi,OFFSET worker mov ax,(Employee PTR [esi]).Years mov ax,[esi].Years ; invalid operand (ambiguous) ; (not clear what type of struct ; “Years” belongs to) Irvine, Kip R. Assembly Language for x86 Processors 7/e, 2015.

Looping Through an Array of Points Sets the X and Y coordinates of the AllPoints array to sequentially increasing values (1,1), (2,2), ... .data NumPoints = 3 AllPoints COORD NumPoints DUP(<0,0>) .code mov edi,0 ; array index mov ecx,NumPoints ; loop counter mov ax,1 ; starting X, Y values L1: mov (COORD PTR AllPoints[edi]).X,ax mov (COORD PTR AllPoints[edi]).Y,ax add edi,TYPE COORD inc ax Loop L1 Irvine, Kip R. Assembly Language for x86 Processors 7/e, 2015.